Testy v0.4.0

Add explicit skipped/incomplete coverage reporting (#36), bounded opt-in full-lifecycle test-case concurrency (#37), and immutable case admission ordering (#38). Steps inside a case remain ordered.

Published from merged master 3a727f6. The release tree matches the tested #38 tree; fresh go test -race -count=3 ./... and go vet ./... passed. Consumer adoption and hosted performance verification remain separate gates.